The present invention is directed to an anchoring device, such as a rock anchor, including a tension member formed by a steel anchor rod having ribs extending along a helical line and forming at least a partial or interrupted thread. An anchor nut is threaded onto the portion of the anchor rod extending outwardly from a body to be anchored and is supported against the body. The nut has at least two axially extending portions, each with an internal thread. Initially, the anchor nut is positioned on the rod with the portions spaced apart at a given distance. Further, the two portions are not rotatable relative to one another. When a predetermined load is exceeded in the anchoring device, the two portions move toward one another and engage in a force-locking manner with the anchor rod, one after the other.
In driving a tunnel, rock anchors together with concrete pumped into place and arc shaped liner plates are finding increasing use as the standard permanent support for lining or at least forming the external shell for the tunnel. This construction method, which has become known as the "New Austrian Tunnel Construction Method" (NOT) also includes careful monitoring of the body or strata overlying the tunnel using measuring technology. Devices suitable for such monitoring usually employ means for checking the anchoring force applied by the anchoring device and are very costly. Accordingly, accurate monitoring can take place only at selected cross-sections of a tunnel, though it would be reasonable for safety to monitor between the measurement cross-sections.
It is known to effect simple visual monitoring of the anchoring force at rock anchor devices by additional steps. Thus, in a rock anchor disclosed in DAS 1 005 474, a spring element having a characteristic corresponding to the desired pretensioning of the anchor, is arranged between the anchor nut and a clamping plate. From the degree of deformation of the spring, it can be determined, as the anchor nut is tightened, whether the anchor has been pretensioned to the desired degree or has loosened somewhat. In a rock anchor, disclosed in U.S. Pat. No. 4,410,296, a special washer, used in place of a spring element, is positioned between the anchor nut and the abutment plate. The washer has finger-like protrusions which project diagonally and at various heights. These protrusions are dimensioned for transmitting the allowable anchoring force, however, they break off one after the other when the anchoring force is exceeded, so that the existence of a possible overload on the anchor can be observed visually.
The common feature of these known monitoring devices is that additional means are used to provide a visual monitoring of the anchoring forces, that is, spring elements or washers are used in addition to the anchor nut required in any case. Such additional means requires increased attention on the part of the personnel installing the anchor devices under difficult conditions for underground rock anchors. Moreover, such means afford only a visual monitoring when the determined anchoring force is exceeded, however, they do not afford any further deformation possibilities. Further, in mining operations, there is the problem that considerable deformation in the overlying strata must be allowed without endangering the stability of the tunneled section. Where the mining is performed at great depths, the pressure of the overlying strata is usually only held temporarily by means of rock anchors, while the strata continue to deform constantly. There is also the problem that deformation of the strata must be permitted to a certain extent so that a new state of equilibrium can be reached after the excavation has been performed. However, there is the problem that the permitted deformation may reach an order of magnitude which exceeds the extensibility of the tension members or anchor rods formed of steel. Accordingly, rock anchors with flexible anchorages have become known whereby relative movement occurs between the anchor rod and the anchoring means when a given axially directed tensile force of the anchor rod is exceeded until the force is no longer exceeded.
In an anchoring device of this type, the anchor nut is formed of a material having a greater strength than the material of the anchor rod, note EP OS No. 0 190 460. The anchor nut is formed so that the pitch of the consecutive threads increases in the direction of the axial tensile force, whereby the flanks of the threads in the nut act on the flanks of the ribs on the anchor rod facing the load with the ribs of the anchor rod being gradually sheared off by the threads on the nut when the axial tensile force is exceeded. In this manner, a flexibility is achieved in the anchor device while maintaining a uniform anchoring force. Such device has the advantage that deformations in the body being anchored are possible to a determined extent, however, there is the disadvantage that the anchor rod, although not impaired with respect to its strength issuing from the core cross-section in the region in which the force transmitting ribs are sheared off, can no longer engage in a form-locking connection with the anchor nut.
To make the anchoring force more uniform, this known anchor nut can be formed of at least two portions threadable engageable with the anchor rod, with the portions in spaced relation to one another on the anchor rod and not rotatable relative to one another. Such portions are axially displaceable by a given distance when a predetermined load is exceeded so that they achieve a force-locking engagement with the anchor rod one after the other. By selecting the spacing between the portions of the anchor nut, which spacing may be smaller or larger than a complete thread turn, the threads of the nut portion can be offset when the portions of the nut contact one another and have attained a force-locking engagement with the anchor rod. Accordingly, it is possible that the curve of slippage resistance, which curve is approximately sinusoidal in a one-piece anchor nut, can be superimposed on one another so that a wave trough, a relieving of one portion of the nut, is superimposed on a wave crest, that is, the complete forcelocking of another portion of the nut, and, as a result, the slippage resistance is made more uniform.